1. Field of the Invention
This invention relates to a permanent magnet rotor in a permanent magnet type rotary electric device such as, for example, a permanent magnet electric motor or a permanent magnet generator, and particularly relates to an embedded magnet type permanent magnet rotor, suited for use in an inner rotor type permanent magnet rotary electric device for effecting decreased torque ripples and cogging torque.
2. Description of the Related Art
A conventional embedded magnet type permanent magnet rotor has been disclosed, for example, in Laid Open Japanese Patent Application Hei-11-262205 or Hei-11-206075, and is illustrated in FIG. 12 herein. The permanent magnet rotor is illustrated in FIG. 12 with respect to the shape of one pole of the rotor in end view. The rotor has a rotor core 1 formed with a plurality of slits 2A, 2B, 2C in multiple layers. Each of the slits 2A, 2B, 2C has an end face in the shape of an arc. The arc is configured such that the longitudinal ends of the arc are located in the vicinity of the outside circumferential surface of the rotor core 1 and such that the longitudinal middle portion of the arc is located radially inwardly of the end portions. Each slit 2A, 2B, 2C extends to the opposite end of the rotor core 1 in the axial direction (i.e., perpendicular to the plane of FIG. 12), and each slit has the same cross sectional shape through its length as the shape of the end face shown in FIG. 12.
In order to form a permanent magnet rotor 10 having a rotor core 1 with permanent magnets embedded, bond magnet (plastic magnet) may be filled in the slits 2A, 2B, 2C and solidified. That is, the bond magnets in the slits 2A, 2B, 2C are formed by injection molding. Preferably, the bond magnet is filled in a magnetic field. Alternatively, permanent magnets are machined respectively in the shapes of the slits 2A, 2B, 2C, and are fitted in the slits 2A, 2B, 2C. As shown in the permanent magnet rotor 10 of FIG. 12, bridges 3 of a certain thickness are formed between the longitudinal ends (portions close to the outside circumferential surface of the rotor core 1) of the slits 2A, 2B, 2C and the outside circumferential surface of the core 1. Thus, the radially outer portions of the rotor core 1 proximate to each slit 2A, 2B, 2C (i.e., the portions of the rotor core proximate to the outside circumferential surface of each slit) and the radially inner portions of the rotor core 1 (i.e., the portions of the rotor core on the center axis side of each slit) will not be perfectly separated by the slits 2A, 2B, 2C. Rather, the bridges provide magnetic paths between the portions of the rotor core on either side of each of the slits 2A, 2B, 2C.
In the conventional permanent magnet rotor 10 described above, the distribution of magnetic flux density formed in the clearance between the rotor and the stator when the rotor is assembled in a rotary electric device is shown in FIG. 13. As illustrated, the distribution of the flux density is in the shape of approximately a rectangular wave, so that the distortion factor is large. The large distortion factor results in increased torque ripples and cogging torque.
The embodiments of the present invention are directed to a permanent magnet rotor that has decreased torque ripples and cogging torque.
One aspect of the present invention is an embodiment of a permanent magnet rotor having a rotor core that comprises a plurality of magnetic poles. The rotor core includes a plurality of permanent magnets embedded in slit sections formed in layers for each magnetic pole. The layers comprise a radially innermost layer, a least one intermediate layer, and a radially outermost layer. The radially innermost layer and the at least one intermediate layer each include at least one vacant slit section in which no permanent magnet is embedded. The vacant section in the radially innermost layer is larger than the vacant slit section in the intermediate layer. The permanent magnets embedded in the slit sections in each layer generate magnetic flux. The permanent magnets embedded in the slit sections of the radially innermost layer generate more magnetic flux than the permanent magnets embedded in the slit sections of the at least one intermediate layer.
In accordance with certain preferred embodiments of this aspect of the present invention in which the vacant slits are provided in the foregoing configuration, the amount of total magnetic flux generated by each permanent magnet layer when the rotor is assembled in a rotary electric device is larger for a radially inward layer than for a radially outward layer. Therefore, the distribution of magnetic flux density formed in the clearance between the rotor and the stator has a stepped shape that is closer to a sinusoidal wave rather than to a rectangular wave. Thus, compared with a conventional permanent magnet rotor, the distortion factor of the waveform of induced voltage is decreased. Torque ripples, cogging torque, noises and vibrations are also decreased.
Another aspect of the present invention is an embodiment of a permanent magnet rotor having a rotor core that comprises a plurality of magnetic poles. The rotor core includes a plurality of permanent magnets embedded in slit sections formed in layers for each magnetic pole. The layers comprise a radially innermost layer, at least one intermediate layer, and a radially outermost layer. The permanent magnets in the radially innermost layer, the at least one intermediate layer and the radially outermost layer are selected to have residual flux densities. The residual flux density of the permanent magnet in the at least one intermediate layer is greater than the residual flux density in the radially innermost layer. The residual flux density of the permanent magnet in the radially outermost layer is greater than the residual flux density of the permanent magnet in the at least one layer. The total magnetic flux generated by the permanent magnet in the radially innermost layer is greater than the total magnetic flux generated by the permanent magnet in the at least one intermediate layer. The total magnetic flux generated by the permanent magnet in the at least one intermediate layer is greater than the total magnetic flux generated by the permanent magnet in the radially outermost layer.
In particularly preferred embodiments in accordance with this aspect of the present invention, at least one of the layers of slit sections has at least one vacant slit in which no permanent magnet is embedded.
In certain preferred embodiments in accordance with this aspect of the present invention, the shape of the distribution of magnetic flux density formed in the clearance between the rotor and the stator can be made closer to a sinusoidal wave than to a rectangular wave, without having any vacant slits, so that magnets can be embedded more efficiently in the rotor core to facilitate size reduction.
Another aspect of the present invention is an embodiment of a permanent magnet rotor having a rotor core that comprises a plurality of magnetic poles. The rotor core includes a plurality of permanent magnets embedded in slit sections formed in layers for each magnetic pole. The layers comprise a radially innermost layer, at least one intermediate layer, and a radially outermost layer. Each layer has a respective mean residual magnetic flux density determined by the kind of permanent magnet in each layer and by a length of the permanent magnet in each layer. At least one of the layers comprises a plurality of slit sections. The plurality of slit sections in the at least one of the layers comprise at least a first kind of permanent magnet having a first residual magnetic flux density and at least a second kind of permanent magnet having a second residual magnetic flux density different from the first residual magnetic flux density. The at least one of the layers has a mean residual magnetic flux density determined by the first and second residual magnetic flux densities and by a length of the first kind of permanent magnet and a length of the second kind of permanent magnet in the at least one of the layers. The mean residual magnetic flux densities of the layers and the lengths of the permanent magnets in the layers are selected so that the mean residual magnetic flux density of the radially innermost layer is smaller than the mean residual magnetic flux density of the at least one intermediate layer, so that the mean residual magnetic flux density of the at least one intermediate layer is less than the mean residual magnetic flux density of the radially outermost layer, and so that a total magnetic flux generated by the permanent magnet in the radially innermost layer is greater than a total magnetic flux generated by the permanent magnet in the radially outermost layer.
In particularly preferred embodiments in accordance with this aspect of the present invention, at least one of the layers of slit sections has at least one vacant slit in which no permanent magnet is embedded. The magnetic characteristics of a permanent magnet layer with a vacant slit is equivalent to that of a permanent magnet layer without a vacant slit in which a different kind of magnet is embedded. Thus, this particular embodiment can facilitate a reduction in the kinds of magnets.
In certain embodiments according to this aspect of the present invention, a plurality of kinds of magnets are embedded in a slit section, so that even if the number of the kinds of available magnets is smaller than that of the slit sections, the permanent magnet rotor is able to produce the same effect as the rotor in accordance with the previously described embodiment of the invention to also facilitate cost reduction.
Another aspect of the present invention is an embodiment of a permanent magnet rotor having a rotor core that comprises a plurality of magnetic poles. The rotor core includes a plurality of permanent magnets embedded in slit sections formed in layers for each magnetic pole. The layers comprise at least a radially innermost layer and a radially outermost layer. Each layer has a respective middle portion and respective longitudinal ends. The layers are spaced apart from each other by respective distances. The distances by which the layers are spaced apart are greater between the respective longitudinal ends of the layers than between the respective middle portions of the layers.
In certain embodiments in accordance with this aspect of the present invention, distances between permanent magnets are differentiated, so that the shape of the distribution of magnetic flux density formed in the clearance between the rotor and the stator when the rotor is assembled in a rotary electric device can be made closer to a sinusoidal wave rather than to a rectangular wave. Therefore, a change in the distribution of magnetic flux density is more moderate compared with a conventional permanent magnet rotor, thereby reducing torque ripples and cogging torque, as well as noises and vibrations.
In preferred versions of the embodiments described above, a first straight line is defined radially outward from a center of the rotor core through a center of the magnetic pole. A second straight line is defined radially outward from the center of the core to a longitudinal end of the permanent magnet in the radially outermost layer. A third straight line is defined radially outward from the center of the core to a longitudinal end of the permanent magnet in the at least one intermediate layer proximate to the longitudinal end of the permanent magnet in the radially outermost layer. A fourth straight line is defined radially outward from the center of the core to a longitudinal end of the permanent magnet in the radially innermost layer proximate to the longitudinal end of the permanent magnet in the at least one intermediate layer. A fifth straight line is defined radially outward from the center of the core midway between the magnetic pole and an adjacent magnetic pole. A first angle defined between the first straight line and the second straight line is greater than a second angle defined between the second straight line and the third straight line. The second angle is greater than a third angle defined between the third straight line and the fourth straight line. The third angle is greater than a fourth angle defined between the fourth straight line and a fifth straight line. In accordance with this version of the embodiments, the shape of the distribution of magnetic flux density formed in the clearance between the rotor and the stator when the rotor is assembled in a rotary electric device can be made closer to a sinusoidal wave, which results in a greater reduction in the distortion factor of induced voltage, torque ripples and cogging torque, as well as a reduction in noises and vibrations.
In further preferred versions of the embodiments described above, each layer of permanent magnets has first and second longitudinal ends proximate to respective locations on an outer circumference of the rotor core. The ends are spaced from the respective locations on the outer circumference by respective bridges. A first pair of straight lines is defined between a center of the rotor core and the longitudinal ends of the radially outermost layer, and a first electrical angle xcex81 is defined between the first pair of straight lines. A second pair of straight lines is defined between a center of the rotor core and the longitudinal ends of the at least one intermediate layer, and a second electrical angle "PHgr"2 is defined between the second pair of straight lines. An nth pair of straight lines is defined between a center of the rotor core and the longitudinal ends of the radially innermost layer, and an nth electrical angle xcex8n is defined between the n th pair of straight lines. The radially outermost layer generates a total magnetic flux "PHgr"1. The at least one intermediate layer generates a total magnetic flux "PHgr"2. The radially innermost layer generates a total magnetic flux "PHgr"n. The following expression of equality between ratios is at least approximately satisfied:
("PHgr"1xe2x88x92(d1 Bs)):("PHgr"2xe2x88x92(d2 Bs)): . . . :("PHgr"nxe2x88x92(dn Bs))=
xcex81 cos(xcex81/4):(xcex82xe2x88x92xcex81)cos((xcex82+xcex81)/4)+xcex81 cos(xcex81/4): . . .
:(xcex8nxe2x88x92xcex8nxe2x88x921)cos((xcex8n+xcex8nxe2x88x921)/4)+(xcex8nxe2x88x921xe2x88x92xcex8nxe2x88x922)cos((xcex8nxe2x88x921+xcex8nxe2x88x922)/4)+ . . .
+(xcex82xe2x88x92xcex81)cos((xcex82+xcex81)/4+xcex81 cos(xcex81/4)xe2x80x83xe2x80x83(1)
where d1 represents the sum of widths of bridges of the radially outermost layer, d2 represents the sum of widths of bridges of the at least one intermediate layer, dn represents the sum of widths of bridges of the radially innermost layer, and Bs is a saturated magnetic flux density of the rotor core. Advantageously, when the expression including the electrical angles xcex81, xcex82, . . . xcex8n and the amounts of total magnetic flux "PHgr"1, "PHgr"2, . . . "PHgr"n is satisfied or approximately satisfied, the shape of the distribution of magnetic flux density formed in the clearance between the rotor and the stator when the rotor is assembled in a rotary electric device is close to a stepped sinusoidal wave which is almost ideal. This results in a substantial reduction in the distortion factor of induced voltage, torque ripples and cogging torque, as well as a reduction in noises and vibrations.
In particularly preferred embodiments, the embedded permanent magnets are formed such that the slits are filled with bond magnet and the bond magnet is then solidified. Advantageously, the permanent magnets are formed by injection molding the bond magnet so that the permanent magnets can be embedded in the rotor core even if the shape of the slits is rather complicated. The injection molding using bond magnet may be an ordinary one in which the bond magnet is filled in the slits and then solidified, or, if the bond magnet is anisotropic, the process may be an in-magnetic field injection molding in which the bond magnet is filled in the slits and solidified in a magnetic field.